1. Field of the Invention
This invention relates to polyoxymethylene with improved resistance to zinc and/or copper ions, to mouldings made therefrom, to a process for the production of polyoxymethylene with improved resistance to zinc and/or copper ions and to the use of polyoxymethylene in contact with corrosive media, for example containing zinc and/or copper ions.
2. Description of the Background Art
Polyoxymethylene (polyacetal) is an outstanding material, from which the most varied practical articles may be produced, in particular by injection moulding. Advantageous properties include in particular chemical resistance to many organic solvents and bases. Since the commercial introduction of polyacetals, many attempts have been made purposefully to improve the properties of polyoxymethylene (POM). Thus, for example, in order to reduce acid lability, which results from unstable semi-acetal end groups on the polyoxymethylene, the polymer end groups have been protected by esterification or etherification (H. Cherdron, L. Hor, W. Kern, Makromol. Chem., 52, 48 et seq. (1962)). It is also known to add thermoplastic polyurethanes in order to improve the impact resistance of POM. However, polyurethanes may also have an acid-stabilising effect at higher contents (DE-PS 11 93 240).
In general, adding thermoplastic polyurethane to polyoxymethylene gives rise to impact resistant polyoxymethylene, as is for example described in EP 0 116 456 B1. POM/TPU moulding compositions are likewise known from DE-OS 37 03 232, in which polyoxymethylene contains 5.3 to 150 parts of a thermoplastic polyurethane and optionally 0.1 to 5 parts, relative to the total quantity of POM+TPU, of alkaline-earth salts of aliphatic mono- to tribasic carboxylic acids with 2 to 20 C atoms, preferably such acids containing hydroxyl groups.
Despite numerous attempts it has, however, hitherto never proved possible to significantly reduce the severe damage caused to polyoxymethylene on contact with zinc and/or copper ions (i.e. so-called non-ferrous metal corrosion). As is known, POM is severely damaged on contact with zinc and/or copper ions. Thus, for example, small standard specimens made from polyoxymethylene immersed in refluxing 0.12 molar zinc chloride solution buffered to pH 5.0 suffer considerable losses of mechanical strength within a few days. Impact testing of such corroded small standard specimens invariably leads to failure before two weeks have passed.
Various additives, such as antioxidants, oxalate, polyethylene glycol or thermoplastic polyurethanes are only capable of slightly retarding non-ferrous metal corrosion under the above-stated test conditions. Thus, for example, the resistance of impact resistant polyoxymethylene, as is known from EP 0 116 456 B1, to zinc and/or copper ions is only slightly improved in comparison with polyoxymethylenes containing no thermoplastic polyurethanes and is in general still unsatisfactory.